The knowledge of channel state information (CSI) at the transmitter side can dramatically improve the performance (data rate, quality of service, spectral efficiency, etc.) of a multiple-input multiple-output (MIMO) antenna communications system. However, especially in case of a multi-user system, it is difficult to achieve full CSI knowledge at the transmitter side. A method for exploiting the multiuser diversity gain with very limited CSI feedback is the opportunistic beamforming technique proposed in P. Viswanath, D. N. C Tse, and R. Laroia, “Opportunistic beamforming using dumb antennas,” IEEE Transactions on Information Theory, vol. 48, pp. 1277-1294, June 2002, which is incorporated herein as a reference. The base station (BS) generates random orthogonal beams (typically the number of beams is equal to the number of antennas at the base station) and each user computes and feedbacks to BS only a parameter that describes their channel quality in each beam, e.g., the signal-to-interference plus noise ratio (SINR) at the output of a linear minimum mean square error (LMMSE) receiver. Based on this information, the BS's scheduler unit associates with each beam the user that can exploit it the most efficiently; therefore, the full multiuser diversity gain is achieved.
It has been shown that in case of systems with very large numbers of users, the opportunistic beamforming performs very close to the optimum beamforming since for each generated beam it is very likely to find a user whose channels are almost perfectly matched. However, the performance deteriorates rapidly for systems with moderate to small numbers of users since, as the number of users decreases, it becomes more unlikely to find users that are almost perfectly aligned with the random generated beams.